1. The Field of the Invention
Embodiments of the present invention relate generally to x-ray devices. More particularly, embodiments of the present invention relate to electron emitters.
2. The Related Technology
The x-ray tube has become essential in medical diagnostic imaging, medical therapy, and various medical testing and material analysis industries. Such equipment is commonly employed in areas such as medical diagnostic examination, therapeutic radiology, semiconductor fabrication, and materials analysis.
An x-ray tube typically includes a vacuum enclosure that contains a cathode assembly and an anode assembly. The vacuum enclosure may be composed of metal such as copper, glass, ceramic, or a combination thereof, and is typically disposed within an outer housing. At least a portion of the outer housing may be covered with a shielding layer (composed of, for example, lead or a similar x-ray attenuating material) for preventing the escape of x-rays produced within the vacuum enclosure. In addition a cooling medium, such as a dielectric oil or similar coolant, can be disposed in the volume existing between the outer housing and the vacuum enclosure in order to dissipate heat from the surface of the vacuum enclosure. Depending on the configuration, heat can be removed from the coolant by circulating the coolant to an external heat exchanger via a pump and fluid conduits. The cathode assembly generally consists of a metallic cathode head assembly and a source of electrons highly energized for generating x-rays. The anode assembly, which is generally manufactured from a refractory metal such as tungsten, includes a target surface that is oriented to receive electrons emitted by the cathode assembly.
During operation of the x-ray tube, the cathode is charged with a heating current that causes electrons to “boil” off the electron source or emitter by the process of thermionic emission. An electric potential on the order of about 4 kV to over about 160 kV is applied between the cathode and the anode in order to accelerate electrons boiled off the emitter toward the target surface of the anode. X-rays are generated when the highly accelerated electrons strike the target surface.
In order to produce high-quality x-ray images it is generally desirable to maximize both x-ray flux (i.e., the number of x-ray photons emitted per unit time) and x-ray beam focusing. An intense x-ray beam is useful for collecting high-contrast images in as short a period of time as possible, while the ability to distinguish between different structures in an x-ray image (e.g., a cancerous mass versus surrounding healthy tissue) is limited by x-ray beam focusing.
X-ray flux can be increased by increasing the number of electrons emitted by the emitter that impinge on the target surface. The number of electrons emitted by the emitter is a function of the area of the emitter and the temperature of the emitter. In general, raising the heating current increases the temperature of the emitter, the increase in temperature increasing the number of electrons emitted by the emitter. In turn, greater x-ray flux is produced when greater numbers of electrons strike the target surface.
While image contrast depends on electron flux, image quality (i.e., the ability to distinguish between different structures in an x-ray image) is a function of the focal spot created by the emitted beam of electrons on the target surface of the target anode. In general, a smaller focal spot produces a more highly focused or collimated beam of x-rays, and a more highly focused beam of x-rays produces better quality x-ray images.
Spiral filaments with circular profiles are problematic because the wide-range of initial trajectories of electrons emitted by such spiral filaments complicates the focusing structures required to focus the electrons into the focal spot on the target surface. Despite the use of such complicated focusing structures, the resulting focal spot still causes the anode to emit an x-ray beam with a double-peaked line shape function, which negatively affects image quality. Further, the resulting focal spot reduces the anode ratability (i.e., the heat input rate capability of the anode) compared to an ideal focal spot, thereby directly affecting the maximum x-ray flux that can be produced by the anode. Finally, the focusing structures for spiral filaments having circular profiles tend to over-focus some electrons, causing areas of x-ray intensity, referred to as “wings,” in undesired locations of the emitted x-ray beam.
The subject matter claimed herein is not limited to embodiments that solve any disadvantages or that operate only in environments such as those described above. Rather, this background is only provided to illustrate one exemplary technology area where some embodiments described herein may be practiced.